A major portion of time spent in medicine is directed toward the problem of diagnosis, and a large proportion of the errors in medicine are made here. A delayed diagnosis raises the level of pain and suffering, and may allow progression to the point of irreversibility; an incorrect diagnosis can be even worse, leading to treatment that is at best unnecessary and at worst harmful or fatal.
Medical imaging, while highly sophisticated, usually merely images body structure without classification into tissue type. For example, an X-ray shows light and dark areas, but it is up to the physician to decide what is "bone" and what is "tissue." Thus, the classification of tissue by type is left to a human decision, or to a posteriori classification rules. A more accurate tissue-type diagnosis usually requires surgical tissue removal (such as biopsy) and subsequent analysis by a pathologist, but still this decision is based upon subjective classification by eye, touch, chemical analysis, or even upon the absorption of exogenous dyes. Currently, it is quite easy to misdiagnose many lesions, as widely different tissues (such as nerves or lymph ducts) may look similar upon first glance.
Light penetrates tissue in small amounts, particularly in wavelengths between 200 nm and 100 .mu.m, with the best deep penetration achieved at wavelengths between 600 nm and 1200 nm. The light that does pass through tissue emerges bearing a signature of the tissue through which it passed, and this signal can be objectively analyzed. Optical methods of monitoring tissue, or invasive methods without optical diagnostics, are taught in U.S. Pat. No. 4,290,433, U.S. Pat. No. 4,622,974, U.S. Pat. No. 4,945,895, U.S. Pat. No. 5,030,207, U.S. Pat. No. 5,131,398, U.S. Pat. No. 5,271,380, and WO 92/17108. Each of these does not perform a tissue analysis, requires fluid or tissue removal or sampling, utilizes fluorescence or other emission-based techniques which measure light other than that used to perform the illumination, is restricted to external or penetrating use, or does not teach tissue classification or identification. Automated classification of tissues for general clinical use via light in vivo has not been taught, nor has such a tool been successfully commercialized.